1. Field of the Invention
The invention relates generally to the field of seismic data processing and interpretation. More specifically, the invention relates to methods for characterizing a hydrocarbon reservoir in the Earth's subsurface using synthetic seismic data adjusted for acoustic Quality factor (Q) determined from well log data.
2. Background Art
Seismic exploration techniques are used to locate subsurface earth formations that are likely to produce economically useful materials such as petroleum. Seismic exploration techniques include deploying one or more seismic energy sources near the earth's surface and deploying an array of seismic sensors at or near the surface in the vicinity of the one or more seismic sources. Seismic energy propagates downwardly from the source, where it may be reflected by subsurface acoustic impedance boundaries. The reflected seismic energy is detected by an array of sensors. The sensors generate electrical and/or optical signals corresponding to the detected seismic energy. The signals are typically recorded for processing.
Seismic processing known in the art includes determining structures of the subsurface earth formations. Typically, structures are inferred by analysis of the two-way travel time of the seismic energy from the source to the various reflective boundaries beneath the surface and back to the sensors at or near the surface.
It is also known in the art to determine various petrophysical properties of the subsurface earth formations by analysis of the frequency content of the detected seismic energy and the phase and amplitude relationships between the seismic energy generated by the source and the seismic energy detected by the sensors. Such analysis includes determining one or more seismic “attributes” of the earth formations. Attributes may be computed prestack or poststack. Prestack means processing prior to summing or “stacking” individual sensor recordings (“traces”) according to a predetermined relationship, such as common mid point (CMP) or common depth point (CDP). Poststack refers to processing after individual sensor recordings have been summed or stacked. Poststack attributes include, for example, reflection intensity, instantaneous frequency, reflection heterogeneity, acoustic impedance, velocity, dip, depth and azimuth. Prestack attributes include moveout parameters such as amplitude-versus-offset (AVO), and interval and average velocities. Further, attributes may be categorized either as instantaneous attributes, wavelet attributes or geometrical attributes. Instantaneous attributes are attributes whose values are obtained for each data point in the seismic data or within a small time window of data points (e.g., a few milliseconds), such as amplitude, phase, frequency and power. “Data points” within seismic data typically refers to numbers each representing a value of seismic trace amplitude at the instant in time at which each of the amplitude values is recorded. Wavelet attributes are the instantaneous attributes computed at the maximum point of the envelope. The physical meaning of all the wavelet attributes is essentially the same as their instantaneous attribute counterparts. Geometrical, or interval, attributes are attributes of a seismic trace within a seismic interval. Interval attributes are computed from the reflection configuration and continuity. The following references describe aspects of seismic attributes and their applications.
U.S. Pat. No. 5,226,019 issued to Bahorich states that with reference to seismic attributes, “combining multiple (i.e. two or more) descriptors through addition, subtraction, multiplication and ratio, or other means can also be successfully employed”, and suggests the use of “a product of the average instantaneous amplitude and average instantaneous frequency.”
U.S. Pat. No. 5,884,229 issued to Matteucci, discloses a statistical method for quantitatively measuring the lateral continuity of the seismic reflection character of any specified location in a subsurface target formation.
U.S. Pat. No. 5,930,730 issued to Marfurt et al., discloses a system for forming a seismic attribute display from calculated measures of semblance and corresponding estimates of true dip and true dip azimuth of seismic traces within an analysis cell.
U.S. Pat. No. 6,012,018 issued to Hombuckle, relates to a system for identifying volumetric subterranean regions bounded by a surface in which a specific seismic characteristic has a constant value. It is stated in the '018 patent that, “in a geological region where physical characteristics (e.g., the presence of oil or gas) are well-correlated with seismic attributes, (e.g., seismic amplitude data), the identification of a subvolume bounded by a constant-seismic-attribute-value surface may provide a very useful predictor of the volumetric extent of the attribute and hence of the characteristic.”
U.S. Pat. No. 5,001,677 issued to Masters, discloses a system, which treats measured attributes derived from seismic data as components of a vector, estimates a background vector representing typical background geologic strata, and then calculates a new attribute. As stated in the '677 patent, the preferred embodiment combines information about P (compressional) and S (shear) impedance contrasts so as to discriminate prospective reservoir strata from surrounding non-reservoir or background strata.
U.S. Pat. No. 5,724,309 issued to Higgs et al, discloses a system in which two new seismic attributes (dip magnitude and dip azimuth) are derived from instantaneous phase. The system comprises determining a spatial frequency value by taking the directional spatial derivative of the instantaneous phase for each of a plurality of x, y, t(z) data points in the seismic data and posting the spatial frequency values to identify changes within the earth's subsurface.
U.S. Pat. No. 5,870,691 issued to Partyka et al., discloses a method for processing seismic data to identify thin beds.
Quality factor (Q) is a measure of how earth formations attenuate and disperse acoustic (seismic) energy. Q has been used as a direct indicator of the presence of hydrocarbons, among other uses. Estimation of attenuation of pressure and shear waves is as important as the estimation of interval velocities in the field of seismic data interpretation. Estimates of attenuation of pressure and shear waves provide an additional perspective of the lithology (rock mineral composition) and reservoir characteristics (rock pore space fluid content, fluid composition, fluid pressure and rock permeability to fluid flow).
An important aspect of seismic survey analysis is characterization of subsurface fluid reservoirs in the Earth over a geographic area for which seismic survey data are available. Reservoir characterization is used in particular to estimate the distribution of fluid content and expected fluid flow properties within the geographic area, and within the various subsurface reservoirs within the area. The accuracy of such characterizations is typically improved when data relating to fluid content and subsurface rock properties are correlated to the seismic data. Such correlations include generation of synthetic seismograms from rock properties inferred from data such as can be obtained by “well logs” (data recorded from instruments lowered into a wellbore drilled through the various subsurface Earth formations). Well log data are used to infer rock mineral content, fractional volume of pore space (porosity) and fractional volume of the pore space filled with connate water and hydrocarbons, as well as an estimate of the density of such hydrocarbons. Methods known in the art for generating synthetic seismograms include estimates of reflection coefficients and placement of subsurface reflection boundaries with respect to depth in the Earth, such that an estimate of how a seismic trace made in the vicinity of the wellbore would appear can be made. Methods known in the art for generating synthetic seismograms from subsurface formation data, however, are not well suited to take into account the attenuation of seismic energy and the spatial distribution of such attenuation. As is known in the art, spatial distribution of seismic energy attenuation can influence analysis methods such as amplitude versus offset (AVO), in which presence of hydrocarbons can be inferred directly from seismic data.
There is a need for methods for generating synthetic seismograms which take into account the attenuation properties of the Earth formations, such that seismic reservoir characterization which makes use of such attenuation properties may be carried out in more precisely